1. Field of the Invention
The present invention relates to simplification and downsizing of a wheel cylinder in a duo-servo type drum brake apparatus mounted in a vehicle or the like.
2. Background Art
There are various types of drum brake apparatus used for braking a vehicle. The drum brake apparatus are classified to a leading trailing type, a two leading type, or a duo-servo type or the like, by an arrangement of brake shoes pressed to an inner peripheral face of a drum substantially in a cylindrical shape.
The duo-servo type drum brake apparatus generally includes a pair of brake shoes of a primary shoe and a secondary shoe arranged to be opposed to each other at an inside of the drum. According to the primary shoe, an inlet side of the drum in a positive rotational direction is made to constitute an inlet portion and an output side of the drum in the positive rotational direction is connected to an inlet side of the secondary shoe byway of an adjuster. Moreover, an outlet side of the secondary shoe is brought into contact with an anchor portion mounted onto a backing plate for receiving an anchor reaction force operated to the primary shoe and the secondary shoe by the anchor portion.
Thereby, when the primary shoe and the secondary shoe are expanded and pressed to the inner peripheral face of the drum, the anchor reaction force operated to the primary shoe is inputted to the input side of the secondary shoe to operate the secondary shoe to press to the drum inner peripheral face, and therefore, both of the primary shoe and the secondary shoe are operated as leading shoes and a brake force having a very high gain can be achieved.
The duo-servo type drum brake apparatus mentioned above is provided with a number of advantages of not only capable of achieving an extremely high brake force, easy to be downsized, and facilitated to be integrated to a parking brake in comparison with a drum brake apparatus of a leading trailing type or a two leading type. However, the duo-servo type is sensitive to a change in a friction coefficient of a lining of the brake shoe, and therefore, the brake force tends to be difficult to be stabilized and a device of stabilizing the brake force is requested.
The inventor has already proposed means for stabilizing a brake force, including a control lever for feeding back a control force in accordance with a control torque to one piston as a wheel cylinder assembly of expanding to open a pair of brake shoes by a pair of pistons in braking, and a control valve for closing a fluid path formed at inside of a cylinder body to restrain a further increase in the brake force when the brake torque exceeds a reference by utilizing the brake force of the brake lever (refer to, for example, Patent Reference 1, Patent Reference 2).    [Patent Reference 1] JP-A-2000-329176    [Patent Reference 2] JP-A-11-303904
Although the brake force can be realized to stabilize by the proposal, the wheel cylinder assembly of the background art forms exclusive cylinder portions for the respective pistons by shifting axial positions on the cylinder body (case of Patent Reference 1 mentioned above), or forms an exclusive liquid chamber for containing the control valve separately from the cylinder portion (case of Patent Reference 2 mentioned above), and therefore, the cylinder body is large-sized.